In the first 4 years of this grant, we assessed the cell biology and transplantation characteristics of adult human oligodendrocyte progenitor cells. We established means for their specific isolation, using CNP2:GFP and A2B5 based fluorescence-activated cell sorting (FACS), and then assessed their lineage potential. We discovered that when removed to low density culture, the progenitors were multipotential, and gave rise to neurons as well as to gila. Thus, absent autocrine and paracrine influences on their differentiation, adult WMPCs were not restricted to oligodendrocytic fate. To assess the role of the tissue environment in regulating progenitor fate, we then investigated gene expression by adult human WMPCs. We focused on identifying those transcripts that are differentially expressed by the WMPC, relative to its local tissue environment. This process enabled us to predict ligand-receptor interactions that maintain the progenitor state, as well as those that determine whether a given cell develops into an astrocyte, oligodendrocyte, or neuron. This analysis identified a set of parallel pathways that together appear to regulate the maintenance, mobilization and differentiated fate of parenchymal progenitor cells. In particular, we noted a complex interaction of: 1) receptor tyrosine phosphatase beta/zeta signaling, as regulated by its ligands pleiotrophin and NrCAM; 2) a parallel avenue of syndecan3 regulated signaling through CASK and tbr1; 3) FGFR3-dependent signaling, likely mediated through syndecan3 proteolysis and release of CASK; and 4) a neuralin and BAMBI-suppression of BMP signals. It appears that the net output of these pathways biases adult progenitors to either self-renewal or differentiation. In this application, we propose to use a combination of protein delivery, adenoviral overexpression and lentiviral RNAi knock-down to evaluate the individual elements of these candidate systems. By this means, we intend to better define the niche for gliogenesis in the adult human white matter, and by so doing to establish both necessary and sufficient genetic targets for directing the phenotypes generated by resident progenitor cells.